The invention relates to the field of splitting and combining radio frequency (RF) signals. More particularly, the invention relates to a switch assembly that selectively combines coherent RF signals to a single RF signal.
Switches and power amplifiers are widely used in electronic apparatuses and systems. One exemplary application for switches and power amplifiers is within a base (transmitter) station that serves in a mobile communications system as an interface between mobile phones, such as cellular phones, and a mobile switching center. Each base station serves the mobile phones within an assigned geographical service area, e.g., a cell in a cellular system, and handles all radio traffic to and from the mobile phones. In order to adequately serve the service area, that is, to reach all mobile stations within the service area (i.e., near or remote), each base station includes a power amplifier section that amplifies a transmit signal prior to feeding it to an antenna of the base station. The power amplifier section is configured to provide an output power that is sufficiently high so that the emitted transmit signal reaches the mobile station with a power level that is sufficient to be detected and processed by the mobile station.
A typical base station includes several individual power amplifier modules that are combined so that the base station meets predefined output power requirements. Outputs of the power amplifier modules are connected to combine amplified coherent signals to one output signal that is provided to the antenna. This technique of combining individual power amplifier modules, rather than using one large power amplifier, has the benefit of increased reliability because each power amplifier module is a complete separate system and can function without any other power amplifier modules operating. But, since the number of combined power amplifier modules is typically four or less, the removal or failure of just one module may cause a significant power loss of the output signal.
For example, a conventional N-way combiner combines the coherent RF signals to a single output signal. If the base station includes four power amplifier modules, the N-way combiner has four input ports connected to the power amplifier modules and one output port coupled to the antenna. A failure of just one of the four power amplifier modules causes the power of the output signal to drop by 50%, and the failure of two power amplifier modules causes the power of the output signal to drop by 75%. This is caused by the limitations of the conventional N-way combiner where any difference of power between the input ports of the N-way combiner is dissipated in its isolation resistors or, in designs without isolation resistors, is reflected back to the power amplifier modules that remain in operation.
Other combiners are based on a different principle. It is known that the impedance at one end of a transmission line having an electrical length that corresponds to 180 degrees (i.e., one-half wavelength) is the same as at the line""s second end. Therefore, when one end of the line is xe2x80x9copen,xe2x80x9d the second end of the line will act also as an xe2x80x9copen.xe2x80x9d This principle applied in the prior art, a mechanical switch is placed exactly 180 degrees from a summing point, which is the point at which the individual amplified RF signals are combined. The position of the mechanical switch with respect to the summing point provides an xe2x80x9copenxe2x80x9d end when the mechanical switch is opened so as to avoid creating an unknown impedance or a short at the summing junction. Individual power amplifier modules can then be removed from the combiner without dramatically effecting the overall performance.
However, one limitation of this principle is that in frequency bands commonly used in mobile communications systems, from around 800 MHz to above 2 GHz, the wavelengths of the signals at these frequency bands are relatively short. This usually requires the transmission lines to have multiple 180-degree sections to meet the mechanical spacing requirements between amplifier modules. This, however, results in a reduced bandwidth, often too small for many applications.
Further, an exact impedance match can only be achieved for a defined number of power amplifier modules. For example, in a design optimized for four power amplifier modules, if two power amplifier modules fail or are removed, the resulting loss, in addition to loss caused by the combiner, is about 0.5 dB or another 12%. With all but one power amplifier module operating, the additional loss increases to approximately 35%. As a compromise, the combiner is, in some cases, configured for an optimum match with three power amplifier modules instead of four power amplifier modules so that the loss is more balanced as power amplifier modules are removed or added.
In order to overcome the impedance problem it is known to switch additional 90-degree (i.e., one-quarter wavelength) transmission lines in and out to maintain a good match regardless of how many power amplifier modules are switched in and out. As the number of power amplifier modules is changed, the impedance of a 90-degree length of the transmission line is changed to maintain a good impedance match under all operating conditions. Such an approach, however, requires many mechanical switches and considerable physical area to achieve a desired performance.
There is, therefore, a need for an improved technique for combining RF signals which is easy to implement and provides for loss-less combining.
An aspect of the invention is therefore a switch assembly having a housing and a plurality of controllable switches. The housing has a plurality of input ports and an output port and the plurality of individually controllable switches is arranged within the housing. Each switch has an open position and a closed position and is coupled to one of a plurality of input ports of the housing and a common summing junction within the housing. Each input port receives a radio frequency (RF) signal. The output port is coupled to the common summing junction and outputs a sum signal that includes at least one of the RF signals received at the common summing junction when a switch is in the closed position.
In preferred embodiments, the switch assembly includes further impedance matching lines that transform a reference impedance value to a higher value prior to switching and an impedance transformation line that provides for the reference impedance value at a distal end of the impedance transformation line. The impedance transformation lines may be located outside the housing, inside the housing or inside and outside the housing. Further, the switch assembly may include at least one matching stub associated with the output port.
Another aspect of the invention involves a switch assembly having a housing, a plurality of controllable switches and a plurality of impedance transformation lines. The housing has a plurality of input ports and an output port. The plurality of controllable switches is arranged within the housing. Each switch has an open position and a closed position and is coupled to one of a plurality of input ports of the housing and a common summing junction within the housing. Each input port is configured to receive a radio frequency (RF) signal, wherein the output port is coupled to the common summing junction and is configured to output a sum signal that includes at least one of the RF signals received at the common summing junction when a switch is in the closed position. A first impedance transformation line is coupled to one of the plurality of input ports of the housing, and a second impedance transformation line is coupled at a first end to the common summing junction. Each first impedance transformation line is configured to transform a reference impedance value at the input port to a higher impedance value, wherein the second impedance transformation line is configured to provide for the reference impedance value at a second end of the second impedance transformation line. At least one matching stub is coupled to the second impedance transformation line.
A further aspect of the invention involves a switch assembly having a housing, a plurality of controllable switches, and a plurality of impedance transformation lines. The housing has a plurality of input ports and an output port. The plurality of controllable switches is arranged within the housing. Each switch has an open position and a closed position and is coupled to one of a plurality of input ports of the housing and a common summing junction within the housing. Each input port is configured to receive a radio frequency (RF) signal, wherein the output port is coupled to the common summing junction and is configured to output a sum signal that includes at least one of the RF signals received at the common summing junction when a switch is in the closed position. A first impedance transformation line is coupled to one of the plurality of input ports of the housing, and a second impedance transformation line is coupled at a first end to the common summing junction. Each first impedance transformation line is configured to transform a reference impedance value at the input port to a higher impedance value, wherein the second impedance transformation line is configured to provide for the reference impedance value at a second end of the second impedance transformation line. At least one internal matching stub is coupled to the second internal impedance transformation line.
Another aspect of the invention involves a method of matching impedances of transmission lines using a switch assembly. The method receives at a plurality of input ports radio frequency (RF) signals and selectively operates switches between open positions and closed positions to selectively couple the RF signals to a common summing junction. The method outputs a sum signal at an output port that includes at least one of the RF signals.
Another aspect of the present invention involves an amplifier section for amplification of a radio frequency (RF) signal. The amplifier section includes amplifier modules, a combiner, first impedance transformation lines and a second impedance transformation line. Each amplifier module has an input port to receive the RF signal and an output port for an amplified RF signal, wherein the input ports is connectable to a splitter. The combiner has input ports each being coupled to an output port of an amplifier module to receive the amplified RF signal and an output port for a single RF signal formed by a combination of the amplified RF signals. The combiner has controllable switches each assigned to one of the amplifier modules to selectively connect and disconnect the amplifier module. The first impedance transformation lines are interconnected at the input ports of the combiner between the combiner and the amplifier modules. Each first impedance transformation line transforms a reference impedance value of a reference line to a first value of an input impedance at the input port, wherein the first value of the input impedance is higher than the reference impedance value. The second impedance transformation line is connected to the output port of the combiner and transforms a second value of an output impedance to the reference impedance value, wherein the second value is lower than the reference impedance value.